1. Field of the Invention
The present invention relates to a toner density sensor for detecting toner density required for image formation suitable for use in color image formation apparatuses, such as color copiers, color printers, or the like.
2. Description of the Related Art
In color image formation apparatuses, such as color copiers, color printers, or the like, a color image is formed by selectively attaching color toner of three colors: yellow, magenta, and cyan to an electrostatic latent image formed on a photoconductor, and the resultant color image is transferred onto a paper sheet or the like. Although the use of such three-color toner allows reproduction of every color including black, to display a black color as vivid as possible, black toner has come to be used in addition to the three-color toner.
Moreover, to achieve satisfactory color reproduction, a toner density sensor is used that detects the density of the toner attached to the photoconductor.
FIGS. 6A and 6B are circuit diagrams illustrating a conventional toner density sensor. The toner density sensor is composed of a light-receiving portion shown in FIG. 6A, and a light-emitting portion and a constant-voltage circuit shown in FIG. 6B. The light-emitting portion is composed of a light-emitting diode LED1 and an LED driving circuit 10. The constant-voltage circuit is composed of condensers C1 and C2, resistances R5, R6, and R7, amplifiers IC5 and IC6, and a regulator REG1.
In the light-emitting portion, after application of power source voltage Vcc, the LED driving circuit 10 drives the light-emitting diode LED1 to emit infrared light. The regulator REG1 and the condensers C1 and C2 are employed to stabilize the circuit, and adjust the resistance values of the resistances R5, R6, and R7 so that the amplifiers IC5 and IC6 output optimal reference voltages Vref1 and Vref2. The light-receiving portion includes a photodiode PD1, a variable resistance VR1, resistances R1, R2, R3, R4, R8, R9, R10, R11, and R12, and amplifiers IC1, IC2, and IC4.
The photodiode PD1 receives light, such as infrared light, and then a voltage is outputted by an I/V (current-voltage) converter constituted by the amplifier IC1 and the variable resistance VR1 and the resistance R12. The output voltage is inputted through the input resistances R2 and R4 to the negative terminals of the amplifiers IC2 and IC4, respectively. The amplifiers IC2 and IC4 have their positive terminals connected to the resistances R8 and R10, respectively, which receive Vref2 and Vref1, respectively. An output of the respective amplifiers IC2 and IC4 is fed back to the negative terminals of the amplifiers IC2 and IC4 through the negative feedback resistances R1 and R3, respectively. At this time, the gain of the amplifier circuit is expressed as R1/R2 and R3/R4.
The light such as infrared light, emitted from the light-emitting diode LED1 of the light-emitting portion is irradiated onto the toner attached to the photoconductor, and is reflected from the toner and the surface of the photoconductor. The resultant reflection light is received by the photodiode PD1 of the light-receiving portion. The photodiode PD1 outputs a current in accordance with the amount of received light, and, in the light-receiving portion, the output current is converted into a voltage by the I/V converter and is then amplified by the amplifier circuit before being outputted. Based on this voltage, the density of the toner attached to the photoconductor is detected.
There is a difference between the amount of variation in output currents for toner density obtained as a result of the detection of color toner density and the amount of variation in output currents for toner density obtained as a result of the detection of black toner density. Therefore, the output current fed from the light-receiving element is, after being converted into a voltage by the I/V conversion circuit provided inside the light-receiving portion, amplified by the amplifier circuit so as to set each variation amount at a predetermined value.
In the toner density sensor disclosed in Japanese Unexamined Patent Publication JP-A 9-89769 (1997), the light-receiving element is arranged in an appropriate position so as not to receive specular reflection light from the surface of the photoconductor. This eliminates the influence of specular reflection light and thus makes it possible to achieve wide-range, highly-accurate detection of color toner density.
The light-emitting diode, the photodiode, and the periphery circuit each have temperature characteristics. As seen from FIGS. 4 and 5 showing temperature characteristics graphs, a voltage to be outputted varies with changes in temperature. FIGS. 4 and 5 are graphs showing temperature characteristics in which the ordinate axis indicates output voltages and the abscissa axis indicates temperature. As the primary temperature characteristics (as observed before compensation), output voltages are high on the lower-temperature side and are low on the higher-temperature side.
Accordingly, even if the amount of received light is kept at a constant level, a variation in output voltage occurs if a temperature change occurs. This makes it impossible to detect toner density with accuracy.
It is an object of the invention to provide a toner density sensor capable of detecting toner density with high accuracy without being affected by a temperature change.
The invention provides a toner density sensor which detects density of toner attached to a photoconductor of an electrophotographic color image forming apparatus based on an output level in accordance with an amount of reflection light of irradiation light irradiated toward the photoconductor, the toner density sensor comprising:
a light-receiving element for receiving reflection light;
an amplifier circuit for amplifying output from the light-receiving element; and
compensation means disposed in the amplifier circuit, for compensating for variation in output level due to a temperature change.
According to the invention, the amplifier circuit for amplifying an output from the light-receiving element includes compensation means for compensating for variation in output level due to a temperature change. This makes it possible to detect toner density with high accuracy without being affected by the temperature change.
In the invention, it is preferable that the compensation means is composed of an input resistance in the amplifier circuit and a negative feedback resistance constituted by a thermistor.
According to the invention, the compensation means is composed of an input resistance in the amplifier circuit and a negative feedback resistance constituted by a thermistor. Therefore, by properly adjusting the value of each resistance, optimal output levels can be attained.
In the invention, it is preferable that an amplifier circuit for detecting color toner density and an amplifier circuit for detecting black toner density are each provided with the compensation means.
According to the invention, an amplifier circuit for detecting color toner density and an amplifier circuit for detecting black toner density are each provided with compensation means. This makes it possible to detect the density of toner, regardless of whether it is color or black toner, with high accuracy without being affected by the temperature change.
According to the invention, by providing a thermistor in each of the amplifier circuit for detecting color toner density and the amplifier circuit for detecting black toner density, of which both amplify outputs from the photodiode, variation in output voltages due to the temperature change can be successfully compensated for. This makes it possible to detect the density of the color or black toner attached to the surface of a photoconductor with high stability and accuracy. Accordingly, a color image forming apparatus in which the toner density sensor embodying the invention is employed is capable of forming an image close to that printed on an original document.